Rinse the elderberries and boil covered with 1.5 liters (approximately 6.25 cups) of water. Let simmer for about 15 minutes and pour through a cloth lined sieve. Drain overnight, but do not press.
Mix the elderberry juice and apple juice to make 1 liter (approximately 4.25 cups). Stir in the sugar. Boil for about 5 minutes. Pour the syrup into jars and seal well. Let cool before serving.